Loss of function mutations affecting proteins required for B-cell activation typically lead to immune deficiency or impairment of antibody production, as in X-linked agammaglobulinemia (XLA) or deletional mutation of CD19. Conversely, mutation of negative regulators, such as the hematopoietic phosphatase SHP-1 (as in motheaten), FAS (as in lymphoproliferative disorder [lpr]) or deletional mutation of CD22, can result in increased production of autoantibodies. This suggests that detailed knowledge of these negative regulatory pathways would aid in the identification of targets for pharmacologic or genetic intervention in auto-immune disorders. Our overall goal is to achieve a better understanding of the mechanisms by which one negative regulator, CD22, functions. CD22, which is expressed exclusively on B cells, is a type 1 transmembrane surface molecule that has been implicated as a negative regulator of the B cell receptor. Functional and biochemical studies indicate that ligation of CD22 alters the signals transduced as a result of B cell receptor (BCR) cross-linking. Deletion mutation of CD22 leads to profound alteration in B cell function that can result in the production of increased amounts of autoantibodies. A combination of genetic and biochemical approaches will be used in a detailed structure/function analysis of this molecule. We will exploit unique tools that we have recently created, which include a transformed B cell line derived from CD22 knockout mice, a CD22 allele-loss variant of the well characterized B cell line WEH1 231, and a CD22 overexpression line. We expect to define the functions of CD22's six cytoplasmic tyrosine residues; determine if the transmembrane and highly conserved juxtamembrane domains of CD22 have specific functions; and determine if the sialic acid binding domains are necessary for function or for the known association with the BCR. We will also characterize molecules that interact with CD22 and participate in the negative regulation of B cell function. An understanding of genes encoding molecules required for negative regulation would aid in future searches for allelic variants that might predispose to autoimmunity.